ICD systems may be used to deliver high energy cardioversion or defibrillation shocks to a patient's heart to terminate a detected tachyarrhythmia, such as an atrial or ventricular fibrillation. Cardioversion shocks are typically delivered in synchrony with a detected R-wave when fibrillation detection criteria are met. Defibrillation shocks are typically delivered when fibrillation criteria are met, and the R-wave cannot be discerned from signals sensed by the ICD. Additionally, ICD systems may also deliver high energy cardioversion or defibrillation shocks to terminate certain types of ventricular tachycardia (VT).
ICD systems generally include an ICD that is coupled to one or more electrical leads placed within or attached to the heart. The electrical leads include one or more electrodes positioned in or on the heart by the leads and used for therapy and/or sensing functions. Cardioversion and defibrillation shocks (e.g., anti-tachyarrhythmia or high voltage shocks) are generally applied between a coil electrode carried by one of the leads and the ICD housing, which acts as an active can electrode.
In addition, or as an alternative to cardioversion and defibrillation shocks, the ICD system may provide pacing therapy to the heart. Conventional ICD systems provide the pacing therapy via the electrodes of the lead that are positioned near or against the cardiac tissue to provide sufficient transmission of electrical energy to the cardiac tissue in order to capture the heart. The pacing therapy may, for example, include cardiac pacing to suppress or convert tachyarrhythmias to sinus rhythm. Such pacing is often referred to as anti-tachycardia pacing or ATP. The ICD system may provide ATP in an attempt to terminate arrhythmias that would otherwise need to be treated by a cardioversion or defibrillation shock, which are uncomfortable for the patient. The ICD system may also provide anti-bradycardia pacing when the natural pacemaker and/or conduction system of the heart fails to provide synchronized atrial and ventricular contractions at rates and intervals sufficient to sustain healthy patient function.